A photovoltaic device converts light into voltage and electrical current. The voltage output of a photovoltaic device depends on its material composition and device structure. Examples of photovoltaic materials include single-crystalline silicon, poly-crystal line silicon, amorphous silicon, CdTe, CuInGaSe, etc., which can be formed in thin films. Device structures include single junction or multi-junction devices. The maximum voltage achieved for open circuit (i.e. zero current) is between 0.2 volts to 5 volts.
An exemplified single junction photovoltaic cell 100, shown in FIG. 1A, includes a transparent upper electrode 110, a PN junction 120 comprising a window layer 130 and an absorber layer 140 that are doped by opposite semiconductor types, a lower electrode 150, and a substrate 155. The transparent upper electrode layer 110 is made of a transparent conductive oxide material. Incident light passing through the upper electrode layer 110 are absorbed by the absorber layer 140, which produces electron and hole pairs. A voltage is generated between the upper electrode 110 and the lower electrode 150, which can produce a photovoltaic current when an electrical load is placed between the two electrodes. The substrate 155 can be made of metallic or insulating material, and can be transparent or opaque.
In another example, referring to FIG. 1B, a photovoltaic cell 160 includes a upper electrode 170, a PN junction 175 comprising an absorber layer 180 and a window layer 185, a lower electrode 190, and a substrate 195. The upper electrode 170 is not required to be transparent. The substrate 195 is made of a transparent material such glass. The absorber layer 180 and the window layer 185 are typically made of oppositely doped semiconductor materials. The lower electrode layer 190 is made of a transparent conductive oxide material. Incident light passing through the substrate 195 and the lower electrode layer 190 are absorbed by the absorber layer 180, which produces electron and hole pairs. A voltage is generated between the upper electrode 170 and the lower electrode 190, which can produce a photovoltaic current when an electrical load is placed between the two electrodes.
The photovoltaic cells are connected in series to increase the output voltage and to reduce internal power loss caused by heating which is proportional to the square of the total current. Each photovoltaic cell can constitute a small portion of a solar power module to minimize the total current generated. A solar power module, for example, can include ten or more serially connected photovoltaic cells. In one implementation, thin-film layers deposited on a substrate of a photovoltaic device are divided into separate photovoltaic cells. The upper electrode of a photovoltaic cell is electrically connected to the lower electrode of an adjacent photovoltaic cell, thereby forming a solar power module comprising serially connected photovoltaic cells.
FIGS. 2A and 2B are respectively cross-sectional and perspective views of an exemplified solar-cell module 200 comprising three serially connected photovoltaic cells 210, 220, 230 on a substrate 205. The photovoltaic cell 210 includes a lower electrode 211 on the substrate 205, a PN junction 212, and an upper electrode 213. Similarly, the photovoltaic cells 220 and 230 include respectively lower electrodes 221, 231 on the substrate 205, PN junctions 222, 232 respectively on the lower electrodes 221, 231, and upper electrodes 223, 233 respectively on the PN junctions 222, 232. The substrate 205 and the lower electrodes 211, 221, 231 can be transparent to allow transmission of incident light to the PN junctions 212, 222, 232. Alternatively, the upper electrodes 213, 223, 233 can be made of a transparent conductive material such as a conductive oxide. The upper electrode 223 in the photovoltaic cell 220 is connected to the lower electrode 211 in the photovoltaic cell 210. The upper electrode 233 in the photovoltaic cell 230 is connected to the lower electrode 223 in the photovoltaic cell 220.
The manufacturing process for the solar-cell module 200 can include depositions of multiple layers for the lower electrodes 211, 221, 231, PN junctions 212, 222, 232, and upper electrodes 213, 223, 233. The layers can be scribed mechanically, by patterning, or by a laser.
One disadvantage of the above described manufacturing process is that a cleaning step is typically needed after each patterning step to remove the debris generated during patterning. Another disadvantage is that the cutting through many layers of the film often causes current leakage between layers and electrical shorting of the photovoltaic cells. Yet another disadvantage of the above described patterning process is that the roughness of the cut or etched surface may lead to lower electrical performance and cause failures in the solar-cell modules. In addition, some conventional solar-cell modules require high transparency for use as windows in buildings. The cost for patterning is high since a large portion of deposited films has to be removed.
The above described disadvantages can increase manufacturing complexity and costs, or decrease the reliability of the solar-cell modules or photovoltaic cells. There is therefore a need for a simpler and more reliable system for manufacturing solar-cell modules or photovoltaic cells.